Port Alberni
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Port Alberni clear sky chart from 2005-11-06 to 2024-01-13.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	19% 12% 4 8% 7% 4 5% 4 5% 6% 26%	74700
	Night	21% 12% 4 8% 6% 4 5% 4 5% 6% 25%	75035
Transparency	Day	59% 8% 11% 14% 7% 1	74383
	Night	61% 8% 11% 13% 6% 1	74870
Seeing	Day	23% 15% 24% 23% 12% 3	14518
	Night	25% 10% 20% 25% 17% 3	22695

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	34% 17% 6% 11% 6% 3 5% 3 3 4 9%	1329
	11th to 20th	Day	23% 16% 7% 12% 10% 3 4 5% 6% 5% 9%	1274
	21st to end	Day	29% 18% 7% 11% 8% 4 3 4 4 3 10%	1553
February	1st to 10th	Day	27% 16% 6% 8% 8% 4 5% 4 4 5% 13%	1620
	11th to 20th	Day	23% 15% 6% 9% 8% 3 5% 5% 4 5% 17%	1575
	21st to end	Day	20% 16% 4 10% 8% 5% 5% 5% 6% 5% 17%	1332
March	1st to 10th	Day	20% 19% 6% 10% 8% 4 5% 4 4 5% 15%	1889
	11th to 20th	Day	24% 16% 6% 9% 8% 4 5% 4 3 5% 16%	1980
	21st to end	Day	25% 18% 6% 10% 7% 4 4 4 3 4 16%	2178
April	1st to 10th	Day	21% 19% 6% 9% 8% 4 5% 4 4 4 16%	2153
	11th to 20th	Day	20% 15% 6% 10% 9% 4 4 4 4 4 19%	2306
	21st to end	Day	22% 13% 4 9% 7% 4 6% 5% 4 5% 20%	2340
May	1st to 10th	Day	18% 11% 4 7% 7% 4 6% 5% 6% 6% 26%	2497
	11th to 20th	Day	16% 12% 4 8% 7% 4 6% 5% 6% 7% 26%	2477
	21st to end	Day	18% 10% 4 8% 7% 4 6% 5% 5% 7% 27%	2885
June	1st to 10th	Day	22% 11% 5% 7% 7% 4 5% 5% 5% 6% 23%	2700
	11th to 20th	Day	16% 12% 5% 10% 7% 4 6% 6% 7% 7% 20%	2700
	21st to end	Day	14% 9% 4 9% 6% 4 5% 4 4 7% 34%	2700
July	1st to 10th	Day	12% 6% 3 6% 6% 4 4 4 5% 6% 43%	2700
	11th to 20th	Day	9% 4 3 5% 4 3 4 4 6% 9% 49%	2700
	21st to end	Day	6% 4 2 4 4 3 4 4 5% 7% 56%	2895
August	1st to 10th	Day	10% 5% 2 4 5% 2 4 5% 5% 8% 50%	2396
	11th to 20th	Day	6% 5% 2 4 5% 2 4 5% 7% 8% 52%	2298
	21st to end	Day	11% 7% 3 6% 6% 3 5% 5% 6% 10% 39%	2574
September	1st to 10th	Day	11% 7% 3 5% 5% 2 4 5% 5% 7% 45%	2169
	11th to 20th	Day	15% 9% 4 6% 6% 3 4 4 5% 6% 38%	2069
	21st to end	Day	19% 9% 4 7% 5% 3 5% 5% 5% 6% 34%	1954
October	1st to 10th	Day	21% 10% 4 8% 7% 4 4 4 5% 5% 30%	1800
	11th to 20th	Day	27% 10% 3 8% 7% 3 5% 5% 5% 7% 19%	1800
	21st to end	Day	27% 10% 4 8% 7% 3 5% 4 5% 6% 21%	1824
November	1st to 10th	Day	31% 15% 5% 10% 8% 5% 5% 4 3 4 11%	1488
	11th to 20th	Day	30% 14% 6% 9% 5% 4 5% 4 5% 4 15%	1520
	21st to end	Day	30% 14% 4 9% 9% 4 4 4 4 4 15%	1520
December	1st to 10th	Day	24% 13% 7% 11% 7% 5% 5% 4 5% 6% 14%	1378
	11th to 20th	Day	31% 18% 7% 15% 7% 3 5% 3 2 3 6%	1685
	21st to end	Day	33% 17% 5% 10% 7% 4 5% 4 4 3 9%	2442
January	1st to 10th	Night	34% 17% 5% 10% 6% 4 4 4 4 4 9%	2836
	11th to 20th	Night	27% 15% 7% 10% 7% 4 5% 4 5% 5% 12%	2730
	21st to end	Night	28% 15% 6% 11% 7% 4 4 4 4 5% 14%	2797
February	1st to 10th	Night	25% 16% 5% 9% 7% 4 4 4 4 5% 17%	2340
	11th to 20th	Night	20% 16% 5% 8% 6% 3 5% 4 4 5% 24%	2272
	21st to end	Night	20% 13% 5% 9% 8% 3 4 4 5% 5% 24%	1924
March	1st to 10th	Night	19% 13% 5% 8% 7% 4 6% 5% 6% 6% 23%	2066
	11th to 20th	Night	19% 14% 6% 9% 7% 4 5% 5% 6% 6% 20%	1979
	21st to end	Night	21% 15% 6% 10% 7% 4 5% 5% 4 5% 18%	2178
April	1st to 10th	Night	18% 13% 5% 9% 7% 4 6% 4 5% 7% 22%	1806
	11th to 20th	Night	16% 11% 5% 8% 7% 5% 7% 5% 6% 7% 23%	1601
	21st to end	Night	21% 10% 5% 8% 6% 4 6% 5% 6% 6% 24%	1620
May	1st to 10th	Night	13% 8% 5% 7% 6% 3 5% 4 5% 6% 37%	1463
	11th to 20th	Night	13% 8% 4 7% 7% 4 6% 7% 7% 9% 27%	1416
	21st to end	Night	13% 8% 4 7% 6% 4 4 5% 7% 9% 34%	1388
June	1st to 10th	Night	18% 8% 4 7% 6% 4 5% 5% 5% 7% 30%	1260
	11th to 20th	Night	13% 6% 3 9% 8% 5% 9% 6% 6% 8% 28%	1260
	21st to end	Night	9% 6% 3 6% 7% 4 5% 6% 7% 8% 38%	1260
July	1st to 10th	Night	10% 7% 2 4 5% 3 4 4 5% 6% 49%	1260
	11th to 20th	Night	8% 4 2 4 5% 2 3 4 5% 10% 54%	1260
	21st to end	Night	6% 3 2 4 3 2 4 4 5% 5% 62%	1420
August	1st to 10th	Night	6% 4 3 4 4 3 3 5% 5% 6% 57%	1541
	11th to 20th	Night	4 3 1 3 5% 3 5% 4 6% 8% 57%	1602
	21st to end	Night	10% 5% 2 6% 6% 3 4 5% 6% 8% 45%	1782
September	1st to 10th	Night	8% 5% 2 5% 4 3 3 5% 5% 8% 52%	1790
	11th to 20th	Night	14% 8% 2 8% 6% 4 5% 6% 5% 6% 37%	1890
	21st to end	Night	19% 8% 3 7% 5% 3 4 4 4 4 38%	2005
October	1st to 10th	Night	19% 7% 3 7% 6% 3 4 4 5% 7% 36%	2160
	11th to 20th	Night	25% 10% 4 7% 6% 3 5% 5% 4 6% 25%	2160
	21st to end	Night	26% 12% 3 7% 5% 3 5% 4 5% 4 25%	2528
November	1st to 10th	Night	30% 14% 5% 8% 7% 3 4 5% 5% 5% 14%	2590
	11th to 20th	Night	29% 14% 5% 9% 6% 3 5% 4 4 4 16%	2660
	21st to end	Night	29% 11% 4 9% 8% 4 4 5% 5% 6% 16%	2660
December	1st to 10th	Night	23% 14% 6% 11% 8% 5% 5% 5% 4 5% 15%	2797
	11th to 20th	Night	32% 18% 6% 12% 6% 3 5% 3 3 4 8%	3520
	21st to end	Night	30% 17% 5% 10% 9% 4 4 3 3 3 10%	5214

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	80% 6% 4 6% 3	1329
	11th to 20th	Day	75% 8% 7% 5% 4 2	1274
	21st to end	Day	77% 6% 7% 6% 3	1553
February	1st to 10th	Day	73% 8% 7% 7% 5% 1	1620
	11th to 20th	Day	69% 7% 10% 10% 4 1	1584
	21st to end	Day	68% 7% 10% 9% 4 2	1332
March	1st to 10th	Day	72% 7% 9% 9% 3	1889
	11th to 20th	Day	73% 6% 9% 8% 4 1	1980
	21st to end	Day	75% 6% 9% 8% 1	2178
April	1st to 10th	Day	73% 6% 9% 8% 3	2153
	11th to 20th	Day	69% 6% 7% 10% 7% 1	2306
	21st to end	Day	67% 8% 9% 11% 5%	2340
May	1st to 10th	Day	59% 10% 12% 15% 5%	2497
	11th to 20th	Day	58% 9% 12% 16% 5%	2480
	21st to end	Day	58% 9% 13% 16% 4	2886
June	1st to 10th	Day	63% 9% 11% 10% 5% 2	2700
	11th to 20th	Day	62% 11% 14% 10% 3	2700
	21st to end	Day	51% 9% 15% 16% 8%	2700
July	1st to 10th	Day	43% 11% 13% 21% 11% 2	2626
	11th to 20th	Day	33% 10% 20% 27% 10%	2619
	21st to end	Day	27% 10% 15% 31% 14% 3	2734
August	1st to 10th	Day	33% 10% 18% 24% 14% 2	2379
	11th to 20th	Day	30% 12% 18% 23% 15% 2	2298
	21st to end	Day	43% 10% 17% 23% 6% 1	2574
September	1st to 10th	Day	38% 10% 13% 26% 11% 2	2169
	11th to 20th	Day	48% 8% 11% 16% 16% 1	2069
	21st to end	Day	52% 8% 14% 15% 11% 1	1954
October	1st to 10th	Day	55% 9% 13% 13% 10% 1	1800
	11th to 20th	Day	64% 10% 11% 12% 3	1800
	21st to end	Day	63% 8% 11% 10% 7% 1	1824
November	1st to 10th	Day	78% 7% 5% 4 4 1	1488
	11th to 20th	Day	71% 8% 8% 6% 6% 1	1520
	21st to end	Day	70% 7% 11% 6% 6%	1520
December	1st to 10th	Day	71% 7% 7% 8% 6% 1	1378
	11th to 20th	Day	79% 7% 8% 4 2 1	1688
	21st to end	Day	80% 6% 7% 5% 2	2442
January	1st to 10th	Night	80% 6% 5% 6% 3	2836
	11th to 20th	Night	73% 8% 7% 6% 3 2	2730
	21st to end	Night	74% 5% 7% 9% 5% 1	2797
February	1st to 10th	Night	71% 7% 8% 7% 6% 1	2340
	11th to 20th	Night	64% 7% 10% 11% 7%	2274
	21st to end	Night	64% 5% 11% 13% 5% 2	1924
March	1st to 10th	Night	62% 8% 13% 12% 5%	2066
	11th to 20th	Night	67% 7% 11% 9% 5% 1	1980
	21st to end	Night	71% 8% 9% 10% 3	2178
April	1st to 10th	Night	63% 9% 12% 10% 5%	1806
	11th to 20th	Night	61% 7% 10% 13% 7%	1601
	21st to end	Night	62% 8% 9% 13% 7% 1	1620
May	1st to 10th	Night	48% 11% 14% 20% 7%	1463
	11th to 20th	Night	52% 13% 15% 15% 4 1	1416
	21st to end	Night	47% 12% 18% 19% 4	1388
June	1st to 10th	Night	54% 9% 18% 12% 5% 2	1260
	11th to 20th	Night	55% 10% 20% 10% 4	1260
	21st to end	Night	42% 11% 21% 17% 7% 2	1260
July	1st to 10th	Night	36% 12% 17% 22% 11% 2	1225
	11th to 20th	Night	28% 10% 25% 26% 10% 2	1218
	21st to end	Night	22% 10% 16% 33% 15% 3	1341
August	1st to 10th	Night	30% 9% 17% 26% 16% 2	1533
	11th to 20th	Night	24% 12% 22% 23% 15% 3	1602
	21st to end	Night	37% 11% 17% 28% 6% 1	1782
September	1st to 10th	Night	31% 11% 16% 26% 13% 3	1790
	11th to 20th	Night	49% 8% 8% 14% 17% 3	1890
	21st to end	Night	50% 7% 12% 19% 10% 3	2005
October	1st to 10th	Night	49% 9% 13% 16% 12% 1	2160
	11th to 20th	Night	61% 8% 11% 13% 7%	2160
	21st to end	Night	63% 7% 8% 13% 8% 1	2528
November	1st to 10th	Night	72% 8% 8% 7% 4 1	2590
	11th to 20th	Night	72% 6% 9% 7% 4 2	2660
	21st to end	Night	68% 8% 9% 9% 5% 1	2660
December	1st to 10th	Night	70% 8% 10% 8% 4 1	2797
	11th to 20th	Night	77% 9% 6% 5% 2 1	3516
	21st to end	Night	80% 4 7% 6% 3	5214

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	38% 17% 16% 18% 7% 5%	180
	11th to 20th	Day	34% 22% 20% 12% 12% 1	164
	21st to end	Day	31% 18% 21% 15% 12% 2	275
February	1st to 10th	Day	38% 19% 24% 14% 5%	340
	11th to 20th	Day	33% 16% 27% 18% 5%	335
	21st to end	Day	26% 25% 24% 19% 6% 2	280
March	1st to 10th	Day	30% 17% 20% 21% 10% 2	398
	11th to 20th	Day	30% 21% 18% 22% 6% 3	420
	21st to end	Day	34% 19% 23% 16% 6% 2	462
April	1st to 10th	Day	33% 20% 23% 16% 7% 1	420
	11th to 20th	Day	27% 21% 23% 17% 9% 3	412
	21st to end	Day	30% 19% 25% 16% 8% 2	420
May	1st to 10th	Day	20% 14% 25% 30% 9% 2	423
	11th to 20th	Day	21% 11% 21% 28% 14% 5%	448
	21st to end	Day	15% 13% 28% 28% 12% 3	647
June	1st to 10th	Day	24% 16% 28% 20% 10% 2	630
	11th to 20th	Day	19% 19% 32% 21% 8% 2	630
	21st to end	Day	16% 13% 25% 30% 13% 3	630
July	1st to 10th	Day	13% 10% 24% 35% 15% 2	621
	11th to 20th	Day	8% 13% 28% 31% 17% 4	618
	21st to end	Day	7% 9% 23% 34% 23% 5%	641
August	1st to 10th	Day	13% 7% 20% 28% 23% 8%	447
	11th to 20th	Day	8% 6% 22% 30% 25% 8%	437
	21st to end	Day	14% 8% 28% 29% 15% 5%	495
September	1st to 10th	Day	16% 10% 28% 30% 12% 4	450
	11th to 20th	Day	22% 11% 23% 26% 14% 4	450
	21st to end	Day	23% 13% 25% 22% 14% 4	450
October	1st to 10th	Day	26% 12% 19% 22% 14% 5%	450
	11th to 20th	Day	32% 12% 20% 19% 12% 5%	450
	21st to end	Day	31% 18% 21% 16% 9% 4	397
November	1st to 10th	Day	34% 21% 14% 21% 9% 1	180
	11th to 20th	Day	32% 14% 17% 16% 14% 6%	180
	21st to end	Day	37% 19% 17% 12% 6% 9%	180
December	1st to 10th	Day	28% 17% 19% 17% 11% 7%	180
	11th to 20th	Day	34% 13% 18% 18% 15% 1	180
	21st to end	Day	40% 15% 17% 18% 9% 1	198
January	1st to 10th	Night	42% 13% 17% 17% 9% 2	850
	11th to 20th	Night	36% 13% 17% 19% 13% 1	810
	21st to end	Night	33% 13% 21% 20% 11% 2	849
February	1st to 10th	Night	32% 14% 24% 21% 8% 1	720
	11th to 20th	Night	27% 14% 22% 25% 12%	704
	21st to end	Night	26% 11% 24% 24% 11% 3	592
March	1st to 10th	Night	24% 13% 24% 21% 15% 3	720
	11th to 20th	Night	27% 17% 23% 20% 12% 1	720
	21st to end	Night	26% 16% 21% 23% 12% 2	792
April	1st to 10th	Night	26% 11% 22% 26% 13% 2	720
	11th to 20th	Night	20% 10% 27% 32% 10% 2	533
	21st to end	Night	24% 8% 22% 32% 12% 2	540
May	1st to 10th	Night	14% 9% 19% 36% 22% 1	383
	11th to 20th	Night	16% 6% 17% 32% 25% 4	354
	21st to end	Night	15% 5% 18% 37% 23% 2	389
June	1st to 10th	Night	18% 9% 22% 28% 23% 1	360
	11th to 20th	Night	15% 7% 24% 36% 17% 1	360
	21st to end	Night	12% 6% 23% 34% 20% 5%	360
July	1st to 10th	Night	11% 4 16% 33% 32% 4	350
	11th to 20th	Night	6% 6% 17% 36% 32% 3	348
	21st to end	Night	7% 5% 13% 31% 40% 5%	394
August	1st to 10th	Night	9% 4 12% 29% 36% 10%	467
	11th to 20th	Night	6% 3 9% 32% 39% 10%	534
	21st to end	Night	13% 4 17% 29% 32% 5%	594
September	1st to 10th	Night	11% 4 17% 35% 27% 7%	710
	11th to 20th	Night	18% 7% 17% 22% 26% 10%	720
	21st to end	Night	22% 10% 18% 26% 20% 4	720
October	1st to 10th	Night	23% 6% 17% 26% 26% 3	720
	11th to 20th	Night	31% 8% 18% 22% 17% 5%	720
	21st to end	Night	30% 10% 22% 23% 12% 2	792
November	1st to 10th	Night	38% 13% 19% 24% 7%	740
	11th to 20th	Night	36% 13% 20% 20% 9% 2	760
	21st to end	Night	34% 10% 21% 19% 13% 3	760
December	1st to 10th	Night	28% 13% 20% 23% 14% 2	825
	11th to 20th	Night	37% 11% 22% 22% 7% 1	850
	21st to end	Night	37% 12% 24% 21% 6%	935

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